C2H4, following the ligand exchange reaction, was observed (route
B). 1–12 were thoroughly characterized.{
when the reaction was carried out in the presence of a two-fold
excess of ethylene (C2H4). Since, under these conditions, the
unlabelled complex 13 could not be detected by 1H NMR
spectroscopy, a C2H4/C2D4 exchange during the reaction can be
excluded. Reaction of the deuterium-labelled pyridine complex
with dppm (molar ratio = 1 : 1) resulted in a yellow product
mixture, in which 13 could not be identified. Recrystallisation from
THF/ether gave some crystals of the Ni(0) complex [(g1-
dppm)2(g2-dppm)Ni] (14), resulting from the reductive decoupling
reaction of the nickelalactone.
Investigations with bisphosphine ligands of the type R2P–
(CH2)n–PR2 demonstrated that the reaction route is strongly
dependent on the chain length of these ligands. For example,
reactions of bisphosphines (where n = 2) resulted in the formation
of thermally-stable five-membered chelate complexes (3–6), while
the homologous phosphines (where n = 3, 4) gave unstable
nickelalactones 7 and 8, which easily underwent a reductive
decoupling to form Ni(0) complexes 11 and 12 (route B) when
warmed up.
An X-ray crystal structure determination of 14 (Fig. 2) revealed
that the Ni atom is tetrahedrally surrounded by four P atoms of
the ligands. Two of them are bound in a monodentate fashion, the
third and fourth act as part of a chelating ligand, yielding a four-
membered chelate ring. In addition, a 31P NMR study showed that
there is an equilibrium between 14 and (dppm)2Ni (15).8 Complex
15 could also be synthesized (together with 13) from the reaction
between dppm, Ni(cod)2 and succinic anhydride (molar ratio =
6 : 3 : 2) under the same conditions that usually result in the
formation of nickelalactones. The hitherto unknown crystal
structure of 15, determined by X-ray analysis, shows that the
two dppm ligands form four-membered chelate rings with Ni(0).{
(iii) When the reaction was carried with a 1 : 2 mixture of dppm
and [(py)2Ni(C2H4COO)] in a solution of DMF, the mixed
complex [(py)(dppm)Ni(C2H4COO)] (16) was isolated as yellow
needles upon partial evaporation of the solvent. Its 1H NMR
spectrum showed that 16 was stable during the measurement at rt.
A small quantity of single crystals were isolated from the mother
liquor, and an X-ray crystallographic study revealed that 16 co-
crystallizes with its partially oxidized product (17), in which the
free P-atom is oxidized to a PO group, in an approximately 75 : 25
ratio. The nickelalactone ring is intact and the dppm ligand is
coordinated in a monodentate fashion. 16 can be considered as the
first reaction product on the way to the binuclear complex 13.
A possible pathway for the formation of 13, based on these
results, is shown in Scheme 3.
Of particular interest, however, was the reaction of the smallest
bisphosphine, bis(diphenylphosphino)methane (dppm, n = 1),
which was found to be the only ligand to react in a complete
different way, affording new compound 13 in high yield (route C,
Scheme 2). Compound 13 is a deep-green crystalline product and
was characterized in solution by NMR spectroscopy and in the
solid state by elemental analyses and single-crystal X-ray analysis
(Fig. 1).
In complex 13, the two Ni(I) centres are linked by a Ni–Ni single
bond (256.3(1) pm) and three different bridging ligands (dppm, the
carboxylato group of acrylate and a diphenylphosphido bridge).
Additionally, each Ni atom coordinates a monodentate dppm
ligand. The bond lengths and angles lie within the range usually
observed for Ni–OCO and Ni–P bonds.
To gain a deeper insight into the course of the reaction to form
13, the following additional experiments were carried out:
(i) Monitoring the reaction between dppm and
[(tmeda)Ni(C2H4COO)] (2 : 1 mixture) by 31P NMR spectroscopy
indicated the formation of Ph2PCH3 (dP = 225.4)
(ii) The deuterium-labelled nickelalactones reacted significantly
more slowly than the ‘‘normal’’ nickelalactone, suggesting that the
b-H–C activation is the rate-determining step for the formation of
13. [(tmeda)Ni(C2D4COO)] reacted under formation of the
deuterium-labelled complex 13. The same product was formed
In a stepwise reaction (via 16) [(dppm)2Ni(CH2CH2COO)] is
formed. This reactive intermediate then undergoes two reactions:
(a) reductive decoupling to form CO2, ethylene and the Ni(0)
species (dppm)3Ni (14), in equilibrium with (dppm)2Ni (15), and
(b) b-H elimination to form a reactive hydrido-Ni(II)-acrylate.
Fig. 1 Molecular structure of complex 13 (H-atoms are omitted for
clarity). Selected bond distances (s) and bond angles (u): Ni1–Ni2
2.563(1), Ni1–P1 2.124(2), Ni1–P2B 2.176(2), Ni1–P1C 2.208(2), Ni1–O2
2.070(4), Ni2–P1 2.128(2), Ni2–P2A 2.193(2), Ni2–P2C 2.205(2), Ni2–O1
2.056(4), O1–C3 1.251(8), O2–C3 1.267(8), C3–C2 1.485(10), C2–C1
1.262(12), Ni1–P1–Ni2 74.14(6), P1–Ni1–P1C 119.15(7), P1–Ni1–P2B
104.15(7), P1–Ni1–O2 113.1(1), P1C–Ni1–P2B 109.91(7), P1C–Ni1–O2
112.6(1), P2B–Ni1–O2 94.7(1), P1–Ni2–P2C 104.85(7), P1–Ni2–P2A
104.85(7), P1–Ni2–O1 118.8(1), P2C–Ni2–P2A 109.03(7), P2C–Ni2–O1
111.3(1), P2A–Ni2–O1 93.5(1).
Fig. 2 Molecular structure of complex 14 (H-atoms are omitted for
clarity). Selected bond distances (s) and bond angles (u) : Ni–P1C
2.1950(6), Ni–P2C 2.1814(6), Ni–P2A 2.1627(6), Ni–P2B 2.1681(6), P1C–
Ni–P2C 77.43(2), P1C–Ni–P2A 117.75(2), P1C–Ni–P2B 112.23(2), P2C–
Ni–P2A 117.07(2), P2C–Ni–P2B 115.86(2), P2A–Ni–P2B 112.34(2).
This journal is ß The Royal Society of Chemistry 2006
Chem. Commun., 2006, 2510–2512 | 2511
Fischer, Reinald
